Police may need the power to detain terrorism suspects for more than 28 days - but the government undermined its own case for an extension to three months by basing it on "three press releases and two sides of A4", a critical report by MPs will say.

In Tony Blair's first parliamentary defeat as prime minister last November, rebel backbenchers and opposition MPs refused to extend the maximum detention period. The Commons home affairs committee is now calling for the creation of an independent body of privy counsellors to advise on the issue, arguing that there is a need to depoliticise such a sensitive matter.

The report, to be released in the next two weeks, is particularly embarrassing for the government as it faces criticism for trying to deal with rows about crime and disorder by making laws on the hoof.

Mr Blair backed the extension of the detention period from 14 days to 90 days within three weeks of the Association of Chief Police Officers (Acpo) proposing the change in a press release. The report suggests the government made its decision on the basis of "three press releases and two sides of A4 describing two operations".

Assistant commissioner Andy Hayman, of the Metropolitan police, told the committee the government had not requested any further evidence or analysis. He accepted the committee's suggestion that the three-month limit was a matter of "instinctive judgment" and stressed that Acpo had been seeking primarily an extension from 14 days, not 90 days in particular.

Deputy assistant commissioner Peter Clarke said anti-terrorist officers reached their conclusion without a formal working party, but on the basis of discussions over several years. He added: "I do not think there is any magic about 90 days."

A source who has seen the report said: "The police put forward the proposal without rigorous analysis. The government then accepted that without saying: this is such a big step, you have to tell us that it will be correctly assessed."

Another source added: "There is no doubt that the management of the arguments undermined the argument itself. A lot of technical witnesses were saying even 90 days might not be long enough and they would like longer. The police, instead of saying 'Just trust us', could have made a more forceful case."

The report also argues that the government should consider allowing intercept evidence to be used in court - as proposed by Acpo - but makes it clear that it may be necessary in addition to extended detention, rather than in place of it. Many critics of the 90-day proposal warned that courts would be reluctant to accept interviews with suspects detained for so long as admissible evidence. But witnesses told the committee that terrorist suspects were unlikely to answer police questions anyway. In many cases, police simply needed more time to gather or analyse evidence.

Several Labour backbenchers were keen for a compromise. They believed Charles Clarke, then home secretary, wanted to reach a deal but was overruled by Mr Blair. Mr Clarke told the committee there had been no prospect of a compromise because the opposition parties were not prepared to move beyond 28 days. It is understood that one member of the committee voted against the report.